I shall use the technique of injecting physiologically identified axons intracellularly with horseradish peroxidase (HRP) to define the terminal morphology of retinal ganglion cell axons in normal and visually deprived adult cats and in normal kittens. The HRP completely stains an axon and permits detailed light microscopic visualization of terminal processes. First, the regions of termination of single, W-, X- and Y-cell axons within the thalamus and midbrain, along with morphologies of terminal zones, will be specified in normal adult cats. Second, axonal arborizations will be characterized in adult cats raised with monocular lid suture. Our initial observations indicate that monocular deprivation causes expansion of X-cell and reduction of Y-cell retinogeniculate terminal fields in the A-laminae of the lateral geniculate nucleus (LGNd). These results suggest competition between X-cell and Y-cell terminations in the LGNd during development, and also suggest retrograde effects of binocular competition as a cofactor. These mechanisms will be evaluated. Third, to directly identify developmental sequences, similar experiments will be initiated in normal kittens. Concurrently, retrogradely labelled retinal ganglion cell somata will be described in the normal and monocularly deprived adult cats and in kittens. Thus, the morphology of entire neurons from cell body to axonal terminations will be characterized. These studies, besides providing basic structure-function correlations of the anatomical organization of physiological pathways at the single-cell level, will provide information fundamental to understanding neural mechanisms of amblyopia. Experiments beyond this grant period will focus on longitudinal effects of visual deprivation on the development of the retinogeniculate pathway. Taken together, these experiments constitute a long-term study of how physiologically defined ganglion cell types distribute information centrally, how these pathways develop, and how the visual environment influences their development.